My Cub Cadet garden tractor will start and run fine for about 20 minutes, then start to stall. If you shut the cutting deck off and turn the throttle down, the engine will stabilize where you increase the throttle so you can move the lawnmower, but if you start the cutting deck it will stall again. After letting the mower sit for 15 minutes you can start cutting grass for another 15 minutes.

This tractor has a 27hp Kohler Courage engine. It seemed to me the engine was being starved for fuel. So I flushed the tank and all the line, replaced the fuel filter, replaced the fuel pump, removed and cleaned the carburetor.
It still doing it. This tractor ran fine for 7 years with normal maintenance. I think I'll replace the spark plug next. Any help would be appreciated.